Here are some things for you to consider.
Ultraviolet light (sunlight) breaks down the lignen at the surface of the wood. Lignen is the stuff in tress that "glues" all of the wood fibers together. As the UV breaks down enough lignen the surface layer starts to "dust" off. This exposes a new layer of lignen to the UV light. Observe old exposed wood that has been dry but exposed to the sun and you'll see that the larger long fibers are still present, but the wood between them has weathered away — the surface lignen is gone and short fibers have been washed/blown away, leaving a wavy grained surface.
When you use clear finishes the UV light passes right through. Some urethanes such as Spar Urethane have UV blockers, but they still don't have a level of UV impermeability that approaches paint. Polyurethane is a clear film finish that lays on top of the wood and adheres to the surface. It doesn't significantly penetrate the wood. Once the top layer of wood that the clear finish is adhered to breaks down the finish is now adhering to dust and the finish begins to delaminate and flake off. Two to three years is about all you will get out of a clear film finish outdoors in the sun before this starts to happen, and it could be as short a a year. A delaminating film finish must be stripped away before refinishing to expose new wood that it can stick to.
A stain will help slow the UV damage (with or without a film top coat) but it doesn't stop it. Oil based stains penetrate the wood and the residual oil in the wood polymerizes. Water based stains will carry the pigments into the wood but when the water dries, unlike oil, it it is gone. Oil based stains have a greater environmental impact while curing, but once cured will be safe for the bees.
Wipe on type penetrating oil finishes get down into the wood, but wipe on application doesn't penetrate deep enough into the wood to offer long term outdoor protection because of water in the wood. A boiled linseed oil or danish oil for example will penetrate and protect the wood well and it will last about two to thee years, but it has to be reapplied. This is an option to seriously consider since it is not a film, it can be reapplied without removing the finish, just clean the dirt off and wipe on new. This easy renewability is why boiled linseed oil is a popular finish for wood fences. It lasts as long as clear films but refinishing is much simpler. (one caution about wipe on oils: practice good rag hygiene so you don't burn down your house or barn - if you've never used oils then google oil rag spontaneous combustion. The way I keep oil finish rags from spontaneously combusting is to let them combust in a controlled manner; I burn the rags in my outdoor fire pit but there are other methods.)
Lightly scorching wood caramelizes some of the wood sugars in the lignen and cellulose fibers. The resulting char, or carbon, surface is UV resistant and also resistant to mold and mildew and insects. A scorched surface needs to be followed with a penetrating oil. I am not a fan of the look but some people love the look.
Hot wax dipped finishes heat the wood enough that it drives the water out and the oils in, giving deep penetration of of heavy oils that will polymerize in the wood as the finish cures. Hot waxed dipped hive bodies will last for many years with no additional top coat. The UV light still harms the wood surface, but the deep wax finish keeps the surface in place (it adheres to it from below) so that layer protects the lignen underneath it.
Paints are film finishes that adhere to the surface of the wood. Since they are opaque UV light cannot get to the surface of the wood. New wood should be primed before painting for longest lasting results. Paint and primer combos are better than no primer, but they still don't approach the performance of a primer layer followed by a paint layer. Paints offer full protection of the wood surface against UV as long as the paint is undamaged and will outperform any clear finish except hot wax dipping. Primer and paint will have a long life of around 15 years and it is easily re-applied. If you are not picky about colors then very high quality exterior paints can be obtained inexpensively from your local Sherwin Williams or Benjamin Moore paint stores if you will ask for mistints (where they goofed on color matching) Tell them what you are using it for and they'll help you out, they are no strangers to beekeepers asking for mistints.
There is a certain appeal to the natural wood finish look for a backyard garden hive. The look comes at a price. As long as you are aware of what will happen and you are OK refinishing every two to three years then go for it.
I paint my hives. I use boiled linseed oil without stain on hive stands as I can wipe on a new coat even with occupied hives sitting on the stands. I also use linseed oil on the cedar shingles of my gabled hive covers and it looks fantastic, and I can wipe on a new coat every year with the cover in place.
